As shown in FIG. 3, a papermaking shoe press apparatus makes use of a shoe press mechanism that a loop-shaped shoe press belt 2 is interposed between a press roll 1 and a shoe 5. Through a press section constructed of the press roll 1 and the shoe 5, a transfer felt 3 and a wet paper web 4 are caused to pass to perform dehydration.
As shown in FIG. 2, the shoe press belt 2 is constructed of a reinforcing fiber base material 6 and an outer circumferential polyurethane layer 21 and an inner circumferential polyurethane layer 22 arranged on opposite sides of the reinforcing fiber base material 6, respectively, such that the reinforcing fiber base material 6 is enclosed (embedded) in the resulting polyurethane layer. Further, a number of concave grooves 24 are formed in a surface of the outer circumferential polyurethane layer 21, the surface being to be disposed on the side of the press roll, such that water squeezed out from the wet paper web 4 upon pressing can be held in the concave grooves 24 and the thus-held water can then be transferred out of the press section as a result of rotation of the belt itself. Therefore, the concave grooves 24 arranged in the outer circumferential polyurethane layer 21 on the side of the press roll are required to be improved in shape retaining properties when pressed between the press roll 1 and shoe 5. In addition, convex areas 25 are also required to be improved in mechanical properties such as cracking resistance, flexing fatigue resistance and abrasion resistance to pressing force applied in a vertical direction by the press roll 1 and friction by the shoe press belt and flexing fatigue in a shoe press region.
For such reasons, polyurethane excellent in cracking resistance and abrasion resistance is widely used as a resin material that forms the outer circumferential polyurethane layer 21 of the shoe press belt 2.
For example, proposed is a shoe press belt formed of a reinforcing fiber base material and a polyurethane integrated with each other, in which the polyurethane is formed of an outer circumferential layer and an inner circumferential layer and the reinforcing fiber base material is embedded in the polyurethane. The polyurethane that forms the outer circumferential layer is a polyurethane, which has a JIS A hardness of 89 to 94 degrees and is obtainable by curing a composition of a urethane prepolymer (“HIPRENE L,” trade name; product of Mitsui Chemicals, Inc.), which is obtainable by reacting tolylene diisocyanate (TDI) with polytetramethylene glycol (PTMG) and has terminal isocyanate groups, and a dimethylthiotoluenediamine-containing curing agent, in which the urethane prepolymer and the curing agent are mixed together in a ratio such that the value of an equivalent ratio (H/NCO) of active hydrogen groups (H) in the curing agent to the isocyanate groups (NCO) in the urethane prepolymer satisfies 1<H/NCO<1.15. The polyurethane that forms the inner circumferential layer is a polyurethane, which is obtainable by curing a composition of a urethane prepolymer (product of Mitsui Chemicals, Inc.), which is obtainable by reacting 4,4′-methylene bis(phenylisocyanate) (MDI) with polytetramethylene glycol (PTMG) and has terminal isocyanate groups, and a mixed curing agent, which contains 65 parts of dimethylthiotoluenediamine and 35 parts of polytetramethylene glycol (PTMG), in which the urethane prepolymer and the curing agent are mixed together in a ratio such that the value of an equivalent ratio (H/NCO) of active hydrogen groups (H) in the curing agent to the isocyanate groups (NCO) in the urethane prepolymer satisfies 0.85≦H/NCO<1 (see Patent Document 1 and Patent Document 2).
Also proposed is a shoe press belt formed of a reinforcing fiber base material and a polyurethane integrated with each other, in which the polyurethane is formed of an outer circumferential layer and an inner circumferential layer and the reinforcing fiber base material is embedded in the polyurethane. The polyurethane that forms the outer circumferential layer and the inner circumferential layer is a polyurethane of a JIS A hardness of 94 to 95 degrees, obtainable by curing a composition of a urethane prepolymer (“HIPRENE L”), which is obtainable by reacting tolylene diisocyanate (TDI) with polytetramethylene glycol (PTMG) and has terminal isocyanate groups, and a dimethylthiotoluenediamine-containing curing agent, in which the urethane prepolymer and the curing agent are mixed together in a ratio such that the value of an equivalent ratio (H/NCO) of active hydrogen groups (H) in the curing agent to the isocyanate groups (NCO) in the urethane prepolymer becomes 0.97 (see Patent Document 3).
Further proposed are a shoe press belt formed of a reinforcing fiber base material and a polyurethane integrated with each other, in which the reinforcing fiber base material is embedded in the polyurethane. The polyurethane of a JIS A hardness of 93 to 96 degrees, which contains a non-reactive and liquid polydimethylsiloxane, is obtainable by curing a composition of a urethane prepolymer, which is obtainable by reacting tolylene diisocyanate (TDI) with polytetramethylene glycol (PTMG), has terminal isocyanate groups and a curing agent, which is selected from dimethylthiotoluenediamine (“ETHACURE 300,” trade name; product of Albemarle Corporation) and 4,4-methylene bis(2-chloroaniline)(“MOCA,” trademark; product of E.I. DuPont de Nemours & Company), in which the urethane prepolymer and the curing agent are mixed together in a ratio to satisfy 0.9≦H/NCO≦1.10. Further proposed a shoe press belt as described above, in which the polyurethane has a JIS A hardness of 90 to 93 degrees and is obtainable by curing a composition of a blended mixture of a first urethane prepolymer, which contains a non-reactive, liquid polydimethylsiloxane and can have a JIS A hardness of 90 to 93 degrees, and a second urethane prepolymer, which not contains non-reactive, liquid polydimethylsiloxane and can have a JIS A hardness of 98 degrees after curing, and dimethylthiotoluenediamine as a curing agent, in which the urethane prepolymer blend and the curing agent are mixed together in a ratio to satisfy 0.9≦H/NCO≦1.10 (see Patent Document 4).
Still further proposed are a shoe press belt formed of a reinforcing fiber base material and a polyurethane integrated with each other, in which the reinforcing fiber base material is embedded in the polyurethane, the polyurethane has a JIS A hardness of 92 to 100 degrees and is obtainable by curing a composition of a urethane prepolymer, which is obtainable by reacting p-phenylene diisocyanate (PPDI) with polytetramethylene glycol (PTMG) and has terminal isocyanate groups, and a curing agent, which contains 85 to 99.9 mol % of 1,4-butanediol and 15 to 0.1 mol % of an aromatic polyamine containing active hydrogen groups (H), in which the urethane prepolymer and the curing agent are mixed together in a ratio to satisfy 0.88≦H/NCO≦1.12; and a shoe press belt as described above, in which the polyurethane has a JIS A hardness of 92 to 99 degrees and is obtainable by curing a composition of a urethane prepolymer, which is obtainable by reacting p-phenylene diisocyanate (PPDI) with polytetramethylene glycol (PTMG) and has terminal isocyanate groups, and a curing agent, which is selected from 1,4-butanediol, hydroquinone bis(β-hydroxyethyl)ether, 3,5-diethyltoluenediamine and 3,5-dimethylthiotoluenediamine, in which the urethane prepolymer and the curing agent are mixed together in a ratio to satisfy 0.88≦H/NCO≦1.00 (see Patent Document 5 and Patent Document 6).
The shoe press belts described in the Examples of Patent Documents 1 to 4 referred to in the above were each so excellent that it developed no crack even after one million reciprocations when its specimen was measured for the number of reciprocations until a crack would have been formed at a reciprocation speed of 40 cm/sec while applying a tension of 3 kg/cm and a pressure of 36 kg/cm2 by an instrument for testing cracking resistance of the type that the specimen was held at opposite ends thereof by clamp hands, the clamp hands were arranged reciprocably in a horizontal direction in an interlocked relation, the specimen was disposed with a surface thereof, which was to be evaluated, directed toward a rotating roll, and a press shoe was moved toward the rotating roll to press the specimen.
The use environment of shoe press belts has, however, become increasingly severer in recent years as a result of increases in operation speed, width enlargements of shoe press belts to about 10 m and higher pressures at press sections, all of which have stemmed from improvements in the productivity of paper. There is hence an outstanding demand for improvements in mechanical properties such as shape retaining properties, especially concave groove-shape retaining properties, cracking resistance, flexing fatigue resistance and abrasion resistance.
Further, the shoe press belts described in the Examples of Patent Documents 5 and 6 referred to in the above were each subjected to a crack forming test under the below-described conditions by using an instrument shown in FIG. 4. As the size of a specimen 41, its width was 60 cm, and the length between grips was 70 mm. By causing a lower grip 42a to undergo a reciprocal motion in a circular arc, an upper grip 42b and the specimen were also reciprocated so that the specimen was flexed and fatigued at a tip of the lower grip. The distance from a center of the circular arc to the tip of the lower grip was set at 168 mm, the distance of a movement of the lower grip was set at 161 mm, and the reciprocation speed was set at 162 reciprocations/min. The weight of the upper grip was set at 400 g. The specimen was repeatedly flexed to determine the number of flexions until a crack was formed. Those shoe press belts developed no crack even after 0.7 million flexions, and therefore, were excellent with improved abrasion resistance.
However, the shoe press belts described in Patent Documents 1 to 6 referred to in the above were not improved in shape retaining properties, especially concave groove-shape retaining properties that affect water squeezability.